repository: add index query function

[ThomasWaldmann]

There are some infos in the repo index which the client might need:

• chunk size
• maybe flags

Scenarios:

• chunks index gets lost on the client side (has id -> refcount, size mapping)
• chunks index missing size info for some chunks

[ThomasWaldmann]

Currently there is:

• .flags and .flags_many to query and/or set flag values
• .list also support mask and value now to limit listing to chunks with specific flags

Also there is the option to call repository.get(..., read_data=False) to only return encrypted metadata of the chunk (which includes size, csize, ...), but this is slower than an in-memory index query as it needs to read from the segment file.