This change adds first-class metadata support to the Azure Blob backend,
including headers, user metadata, tags, and modtime overrides, and wires
it through uploads and server-side copies.
There is a behavior change in that rclone will now set the "mtime"
custom metadata when doing server side copies to azure and the
`--metadata` argument is given.
- Map standard headers: cache-control, content-disposition, content-encoding,
content-language, content-type to corresponding x-ms-blob-* HTTP headers.
- Map user metadata: any non-reserved keys (excluding x-ms-*) are sent as
blob user metadata. Keys are normalized to lowercase for consistency.
- Support tags: parse `x-ms-tags` as a comma-separated list of key=value
pairs and apply them on uploads and copies.
- Support mtime override: accept `mtime` in metadata (RFC3339/RFC3339Nano)
to override the stored modtime persisted in user metadata.
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