Add documentation for preprocessors

[jrhee17]

Notes

• This PR serves as a design proposal for the new preprocessors API and will be merged once features regarding preprocessors are fully implemented.
• See Poc/xds filter v2 jrhee17/armeria#36 for details on proposed changes

[github-actions]

🔍 Build Scan® (commit: 0f48096)

Job name	Status	Build Scan®
build-ubicloud-standard-8-jdk-8	✅	https://ge.armeria.dev/s/4vawotrmypky6
build-ubicloud-standard-8-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/76iefvbt3nazc
build-ubicloud-standard-8-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/ohl6n5tmdnqi2
build-ubicloud-standard-8-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/yffonhfasko6y
build-ubicloud-standard-8-jdk-17-leak	✅	https://ge.armeria.dev/s/rcd7mvqxua7ai
build-ubicloud-standard-8-jdk-11	✅	https://ge.armeria.dev/s/4ypuwyxrtbfte
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/cfrudzdrelc2w

[minwoox]

The new interface looks promising. 👍 👍 👍

[minwoox]

I also understand that the decorator chain is mutable so that different decorators can be added in the preprocessor.
e.g. Different Retrying client based on the RouteConfiguration.

[ikhoon]

👍👍

[ikhoon]

I can understand what Partial means, but it might be ambiguous for normal users.
What do you think of prefixing Pre to imply it is used in *Preprocessor?

Suggested change

	<type://HttpPreprocessor> and <type://RpcPreprocessor> expose a <type://PartialClientRequestContext>.
	<type://HttpPreprocessor> and <type://RpcPreprocessor> expose a <type://PreClientRequestContext>.

[ikhoon]

👍