feat(client-reports): Client report protocol

[szokeasaurusrex]

Added the client report protocol.

Resolves #1001
Resolves RUST-153

[github-actions]

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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[linear-code]

RUST-153

[szokeasaurusrex]

These docs.rs links of course do not work yet, but should be accurate after release.

[lcian]

LGTM, please address bot comment as that looks legit, we would have anyways found that out when adding the category and de/serialization tests.

[sentry]

Bug: The Report struct's timestamp field will fail to deserialize if the JSON value is explicitly null, because the custom deserializer does not handle null values.
_{Severity: HIGH}

Suggested Fix

Modify the custom deserializer to handle null values. Implement visit_none in the SecondsTimestampVisitor to return a default value, or wrap the deserialization logic in a function that handles an Option<T> to correctly process null before it reaches the custom timestamp logic.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: sentry-types/src/protocol/client_report/mod.rs#L21-L22

Potential issue: The `Report` struct uses `#[serde(default = "SystemTime::now")]`
combined with a custom deserializer `#[serde(with = "utils::ts_seconds_float")]` for its
`timestamp` field. The `default` attribute is only triggered when the field is missing
from the JSON, not when it is explicitly `null`. If a client report is received with
`{"timestamp": null}`, `serde` will attempt to deserialize the `null` value using the
custom `ts_seconds_float` deserializer. This will fail because its internal visitor,
`SecondsTimestampVisitor`, does not implement `visit_none()` to handle `null` values,
causing the entire report to be dropped.

Also affects:

• sentry-types/src/protocol/v7.rs:751
• sentry-types/src/protocol/v7.rs:895
• sentry-types/src/protocol/v7.rs:1143