Enforce string limits for deserialization

[zbuc]

Describe your changes

This follows the suggestion from @hdevalence in #3620 (comment) to add validation of user-supplied String lengths to the various TryFrom protobuf deserialization implementations.

The user-supplied String values concerned are found within the governance and staking crates, concerning governance proposals and validator definitions.

This is consensus breaking as historic data may not comply with the new limitations. Thus there may be unexpected panics when trying to retrieve and deserialize data from storage.

An additional migration has been included to truncate any existing data that does not adhere to the new limits.

The strings are configured to use byte length limits rather than character length limits. This seems favorable because values will be guaranteed to fit into fixed-size buffers of the byte length limit for the field. Code based on rust-lang/rust#93743 was pulled in to perform the truncation at UTF-8 character boundaries.

Issue ticket number and link

Closes #3620

Checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

[erwanor]

Relative to the proposed migration structure, the limits lgtm