Add implementation specific attributes

[collinoc]

Which issue does this PR close?

Closes #446.

Rationale for this change

Currently there seems to be no support for applying implementation specific attributes within this API. For example with a aws_store.put_opts(...) call, there would be no way to specify a x-amz-object-lock-mode attribute, or applying x-goog-storage-class for a GCP put operation. This adds support for such use cases.

What changes are included in this PR?

Expands the Attribute enum to include a Other variant that will add headers with implementation specific prefixes (x-amz- for AWS, etc.)

Are there any user-facing changes?

Yes, documentation was updated where I thought needed. Let me know if more is needed.

These changes should be backwards compatible.

[tustvold]

Whilst I can appreciate the flexibility this would offer, I am somewhat concerned about the potentially hard to reason about interactions with other attributes... In particular if I specify Attribute::Other("meta-") the outcome will end up implementation defined based on whether ${IMPL_DEFINED_ATTR_HEADER_PREFIX}-meta- happens to map to Attribute::Metadata.

Similarly this somewhat breaks the design principal that attributes should roundrip or be rejected...

I wonder if we might instead just add support for the attributes you wish to support, i.e. lock mode and storage class?