refactor: Refactor FDV2 protocol handling.

[kinyoklion]

This PR introduces a few refactoring changes to:

• Better align FDv2 code with the java/dotnet implementations.
  • The protocol handler is its own entity.
  • The word "basis" is reserved for the header we use in the FDv2 protocol instead of being overloaded to represent a full set of data.
• Better align the FDv2 code with the client-side requirements for JS in this repo. (Reducing usage of classes and other features that increase code size.)

Likely subsequent changes will be made to use a more "pull" based approach to the data sources instead of the callback based approach, but that is TBD.

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 171486 bytes
Compressed size limit: 200000
Uncompressed size: 798889 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 23601 bytes
Compressed size limit: 25000
Uncompressed size: 81778 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 19331 bytes
Compressed size limit: 20000
Uncompressed size: 99578 bytes

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25337 bytes
Compressed size limit: 26000
Uncompressed size: 124159 bytes

[kinyoklion]

I am not convinced this is the best layer for this. I think it may be better one level up.

[kinyoklion]

Another PR will propagate the changeset type through the layers. I prefer an actual interface for this instead of a single field.

Like this: https://github.com/launchdarkly/java-core/blob/823d276b7e3fb9d0199ff091316fb4199e849a6e/lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/subsystems/DataStoreTypes.java#L396