chore: refactorings for CreateMultipartUpload

[BenWhitehead]

1. Define Codec<OffsetDateTime, String> to encapsulate the logic and use of date time formatter
   • Use existing RFC3339 formatter as it's already used by json and tested to be valid
   • update tests to assert equality of OffsetDateTime instances instead of strings -- this ensures that equivalent times but different strings are valid
2. Refactor the boostrapping of MultipartUploadHttpRequestManager to be a factory method that only needs HttpStorageOptions
   • This removes the need for tests and implementation to perform manual bootstrapping multiple times unnecessarily
   • It also removes the boostrapping logic from MultipartUploadClientImpl and in turn removes the need for HttpStorageOptions to be passed around to multiple places
3. pre-compute and have a single stable location for all headers which apply to all requests -- x-goog-api-client, x-goog-user-project, user-agent, etc
   • This makes it so fewer things need to know about and interact with HttpStorageOptions, thereby simplifying multiple methods
   • The logic and resolution happens once per client instance removing the need for redundant string concatenations, or environment probes
   • leverages existing abstractions for representing headers, and plumbs user provided headers from HttpStorageOptions
4. Add logic to compute x-goog-api-client header to match what apiary does
5. Update request header processing to avoid allocating a temporary map, instead add directly to the requests headers.