feat: add etag for objectMeta

[Weijun-H]

Which issue does this PR close?

Closes #2240

Rationale for this change

Add Etag for objectMeta as a unique identifier, which is a hash value by last_modified.

What changes are included in this PR?

Are there any user-facing changes?

[tustvold]

For the actual stores, like S3, the etag is part of the response. Using a manual hash is just for in memory and filesystem where this data isn't stored

[tustvold]

The emulator tests appear to be failing for this?

[Weijun-H]

The emulator tests appear to be failing for this?

---- http::tests::http_test stdout ----
thread 'http::tests::http_test' panicked at 'called `Result::unwrap()` on an `Err` value: Generic { store: "HTTP", source: InvalidPropFind { source: Custom("missing field `getetag`") } }', object_store/src/lib.rs:1203:62

getetag should work, but it failed. I am not idea how to change it.

[13.6](https://datatracker.ietf.org/doc/html/rfc2518#section-13.6) getetag Property

   Name:       getetag
   Namespace:  DAV:
   Purpose:    Contains the ETag header returned by a GET without
   accept headers.
   Description: The getetag property MUST be defined on any DAV
   compliant resource that returns the Etag header.
   Value:      entity-tag  ; defined in [section 3.11 of [RFC2068]](https://datatracker.ietf.org/doc/html/rfc2068#section-3.11)

   <!ELEMENT getetag (#PCDATA) >

[tustvold]

It may not be supported, we may need to make the ETag optional 🤔

[tustvold]

Looks good to me, thank you, as this is a breaking change I'm going to hold off on merging this until I've cut the next point release which I intend to do this wewk