Make zenoh key expression structure easier to maintain

[guzman-raphael]

Features

• Localize zenoh key expression structure to a single builder for both pub/sub (see make_key_expr). Keys are alphabetically sorted in key expression.
• Relax structure to be mostly self-describing. This means that the builder structure is minimal and the caller is free to add as much context as necessary via metadata.
• When in sub mode, key are delimited by wildcards to allow room to grow. The result is a key expression parsing logic that will be backward compatible with future releases provided we primarily add new keys in the future. Renames will present backward incompatibility but that is somewhat expected. Therefore to avoid this, we should try to pick good key names that we don't expect to rename in the future.
• Eliminate the need to maintain an agent regex and the capture struct i.e. EventMetadata.

Tests

• Add a default and a test feature. It is only allowed to enable one of them at a time.
  • Default (which is the default feature) will not expose any part of the private API in core.
  • Test will expose some portion of core. This is useful when integration tests need access to core.

Docs

• Add note about using git worktree

Housekeeping

• Simplify errors related to None values to a single MissingInfo error. The details field can be used to elaborate on what is missing and provide a hint to API user.
• Clean up some of the while loops that silenced certain error states.

[Synicix]

Merge to have a base to work off of