feat(replays): Add additional safety to tags processor

[cmanallen]

closes: https://github.com/getsentry/replay-backend/issues/194

• Add upper limit to the number of tags accepted per request.
• Enforces non-null string keys.
• Enforces nullable string values.
• Preserves duplicate tag keys (supported by the replays product).
• Skips (instead of erring on) malformed tag tuples.

[evanh]

Preserves duplicate tag keys (supported by the replays product).

While you can certainly store duplicate tag keys, I don't know that this will actually work in practice. Queries against the tags in Snuba are transformed into something equivalent to tags.values[indexOf(tags.keys, <value>)]. If you are storing duplicate tag keys, you will only ever get the first value in the array when you query the tags.

Otherwise this looks fine to me.

[cmanallen]

@evanh Replays uses its own tags filtering technique so this shouldn't be a problem. We display tags like {"key": [value1, value2]} so we wrote filters to accommodate that.