Check all env vars and suggest alternatives for misspelled keys

[DL6ER]

What does this implement/fix?

This PR adds checking for all FTLCONF_* environment variables. We log used ones, invalid ones (e.g. 123 for something expecting a boolean), and unknown keys (ignored). In the latter case, we try to suggest a meaningful alternative to aid typo fixing.

Example environment:

FTLCONF_misc_nice=-12
FTLCONF_misc_privacyllll=-12
FTLCONF_dns_prt=123
FTLCONF_debug_api=not_a_bool
FTLCONF_dns_upstrrrr=1,2,3
FTLCONF_debug_ApI=true

leading to the following log output:
(removed, see updated output in comment below)

When no environment variables are defined, nothing environmental variables-related is logged at all.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

---

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[yubiuser]

FTLCONF_debug_api is invalid, using default

Would be nice if the log would tell us, why it is invalid. E.g.

FTLCONF_debug_api is invalid (expecting bool, string set), using default

[DL6ER]

The latest commits change the output to

2023-11-01 06:52:11.637 [1067602M] INFO: 6 FTLCONF environment variables found (1 used, 1 invalid, 4 ignored)
2023-11-01 06:52:11.637 [1067602M] WARNING: [?] FTLCONF_debug_ApI is unknown, did you mean any of these?
2023-11-01 06:52:11.637 [1067602M] WARNING:     - FTLCONF_debug_api
2023-11-01 06:52:11.637 [1067602M] WARNING: [?] FTLCONF_dns_upstrrrr is unknown, did you mean any of these?
2023-11-01 06:52:11.637 [1067602M] WARNING:     - FTLCONF_dns_upstreams
2023-11-01 06:52:11.637 [1067602M] ERROR:   [✗] FTLCONF_debug_api is invalid (not of type bool), using default
2023-11-01 06:52:11.638 [1067602M] WARNING: [?] FTLCONF_dns_HOSTS is unknown, did you mean any of these?
2023-11-01 06:52:11.638 [1067602M] WARNING:     - FTLCONF_dns_hosts
2023-11-01 06:52:11.638 [1067602M] WARNING: [?] FTLCONF_misc_privacy is unknown, did you mean any of these?
2023-11-01 06:52:11.638 [1067602M] WARNING:     - FTLCONF_misc_privacylevel
2023-11-01 06:52:11.638 [1067602M] WARNING:     - FTLCONF_misc_nice
2023-11-01 06:52:11.638 [1067602M] INFO:    [✓] FTLCONF_misc_nice is used

Furthermore, we extend suggesting mistyped keys to the CLI (pihole-FTL --config) as well:

$ pihole-FTL --config dbg.all
Unknown config option dbg.all, did you mean:
 - debug.all

$ pihole-FTL --config deg.t
Unknown config option deg.t, did you mean:
 - debug.tls

$ pihole-FTL --config dns.upsrrrrr
Unknown config option dns.upsrrrrr, did you mean:
 - dns.upstreams

$ pihole-FTL --config misc.privaaaacy
Unknown config option misc.privaaaacy, did you mean:
 - misc.privacylevel

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[DL6ER]

Rebased on latest development-v6

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[PromoFaux]

pihole  | 2024-02-10 08:25:00.174 [146M] WARNING: [?] FTLCONF_webserver_aip_passwordddd is unknown, did you mean any of these?
pihole  | 2024-02-10 08:25:00.174 [146M] WARNING:     - FTLCONF_webserver_api_password