Code coverage preparations (#4387)

Added manual jobs for code coverage (triggered via `codecov-start` job):
 - **codecov-start** - initialize Codecov report for commit/pr
- **test-linux-stable-codecov** - perform `nextest run` and upload
coverage data parts
- **codecov-finish** - finalize uploading of data parts and generate
Codecov report

Coverage requires code to be built with `-C instrument-coverage` which
causes build errors (e .g. ```error[E0275]: overflow evaluating the
requirement `<mock::Test as pallet::Config>::KeyOwnerProof == _\` ```,
seems like related to
[2641](https://github.com/paritytech/polkadot-sdk/issues/2641)) and
unstable tests behavior
([example](https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6004731)).
This is where we'll nee the developers assistance

closing [[polkadot-sdk] Add code coverage
#902](https://github.com/paritytech/ci_cd/issues/902)