Add support for parsing echcheck related metadata in pipeline

[hellais]

We have added a new test called echcheck that's shipped since November 2024 in all probes, see: https://github.com/ooni/spec/blob/master/nettests/ts-039-echcheck.md.

Currently the data pipeline doesn't support parsing this new test. We should do that by implementing a new model for the test (see: https://github.com/ooni/data/blob/main/oonidata/src/oonidata/models/nettests/web_connectivity.py) and then adding a transformed to fill out the tls_echconfig field: https://github.com/ooni/data/blob/main/oonidata/src/oonidata/models/observations.py#L287 in here: https://github.com/ooni/data/blob/main/oonipipeline/src/oonipipeline/transforms/nettests/web_connectivity.py and then adding it to the transforms list: https://github.com/ooni/data/blob/main/oonipipeline/src/oonipipeline/transforms/observations.py#L43.

An example on how this is done can be seen in this PR: #50.

Note: since then some slight changes to the structure of this repo might have changed, but the core logic stays more or less the same.

[hellais]

This is done