Remove num_enum dependency

[muzarski]

Ref: #771
I believe this PR closes #857.

Motivation

To stabilize our API, we need to get rid of pre-1.0 crates. One of them is num_enum.

This crate allows us to automatically generate conversions from primitive types to enum values - however, its usages needs to be removed from the library.

Changes

To get rid of the dependency, we needed to implement converters ourselves.

This is why, we introduce a TryFromPrimitiveError struct which holds the information about the enum name and the actual value that we tried to parse.

Apart from that, we implement TryFrom for the types that used the num_enum crate:

• impl TryFrom<u16> for Consistency
• impl TryFrom<i16> for SerialConsistency
• impl TryFrom<u8> for RequestOpcode
• impl TryFrom<u8> for ResponseOpcode

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.

[piodul]

Two tiny nitpicks, otherwise LGTM.

[muzarski]

v2: addressed review comments

[piodul]

Cassandra tests failed on a known flaky test: #813. Rerunning CI.