session: include partition key in RequestSpan in human readable form

[piodul]

This PR changes the format used by RequestSpan to represent the partition key for a given query. Previously, it was the same representation that the driver creates before applying MurmurHash3 to it, encoded in base64 - the format is documented so technically users can decode it themselves, but it's quite inconvenient. Now, the driver decodes the contents of the partition key and represents it in a human-readable form, similar to the CQL syntax.

For example, given the schema

create table ks.t (pk1 int, pk2 time, pk3 frozen<set<text>>, primary key(pk1, pk2, pk3));

and the request

session
    .execute(
        &prepared,
        (
            6622i32,
            Time(chrono::Duration::seconds(12323)),
            vec!["quick", "brown", "fox"],
        ),
    )
    .await
    .unwrap();

previously, the encoded key would look like this:

partition_key=0004000019de00000800000b352c099e0000001d0000000300000005717569636b0000000562726f776e00000003666f7800

but now it looks like this:

partition_key=6622,'03:25:23.000000000',{'quick','brown','fox'}

Fixes: #720

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[wprzytula]

Looks good, only some naming/comments nits.

[piodul]

v2:

• std::iter::chain instead of collect to vector then push,
• Rename idx -> col_idx,
• Rename index -> value_index,
• Added a link to the explanation of the format that PartitionKeyDecoder decodes from,
• Reworked the tests for PartitionKeyDecoder a bit and added a more complicated case that involves out-of-order columns and different column types.