feat: typesafety for client options (new surface only)

[bshaffer]

Add classes to wrap our current typeless array for Client Options. There is a different "Options" class for each "layer" of options (e.g. nested array), so that we have typehinted classes with corresponding docs instead of untyped associative arrays. This will allow us to have consistent options across our various classes, including handwritten clients.

An overview of the design decisions

1. I implemented ArrayAccess in OptionsTrait so that the getters do not show up in the docs or IDEs. This is just a nice convenience factor, since the getters shouldn't be used or be considered very important. It also causes less churn for our internal code, since we were previously using array access.
2. I made all the setters public so they do show up in IDEs and docs. These could be made private, as the most common API will be to just call the constructor with an array. But I thought it seemed nicer for the documentation and API.
3. I didn't add typehints for the Options classes public methods (e.g., customers could only use these classes in their code if they called toArray() before passing them anywhere). This is because I didn't want to widen (e.g. remove) the array typehints (as union types aren't available until PHP 8.0! 😡). If we decide to do it this way (we can always widen later), we should at least add a reference (e.g. @see ClientOptions), etc, to everywhere it's applicable, so users can find the docs.
4. I left all strings nullable. We could default to empty strings, but I feel like null says something different. I did not allow arrays/booleans to be nullable, because I don't think that distinction means anything (and it just makes the code look sloppier).

cc @saranshdhingra and @noahdietz - This is a draft! I am not hard-set on any of these design decisions! I would still love your feedback!

• Only use in V2 client surfaces
• Add CallOptions
  • See misc-config-options and group-grpc-args
• Add tests
• Add better docs explaining its usage
• Get design approval
• drop support for PHP 7.4

[bshaffer]

Added Do not merge as I'd like to wait one week after we release #470 in order to mitigate churn