Implement Serialize for lambda telemetry

[nismotie]

Issue #757 Lambda Extension - Implement Serialize for LambdaTelemetryRecord

Description of changes:
This PR implements the Serialize trait for the LambdaTelemetry struct, as well as its fields, most importantly the LambdaTelemetryRecord enum and its component variants.

I have made a couple choices here and I'm not sure that they are necessarily the best, I'm happy to hear any comments on them:

• On variants of LambdaTelemetryRecord, I have skipped serialization on fields which are Option::None, as opposed to serializing them to null
• I have included empty arrays of spans on those variants that have spans

By submitting this pull request:

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[calavera]

• On variants of LambdaTelemetryRecord, I have skipped serialization on fields which are Option::None, as opposed to serializing them to null

• I have included empty arrays of spans on those variants that have spans

both of these options sound reasonable to me, thanks!