Add additional validations for spans

[the-ericwang35]

Adding in additional validations for tracing based on this doc. The additional checks are

• Validating the project name and span names are in the correct format
• Validating that strings and truncated strings do not exceed the maximum lengths
• Validating that optional fields (attributes, time events, links, etc.) follow the correct format if present

Additional unit tests for these validations will be added in a separate PR since this one is already pretty large.

Since this involved creating a lot of new statuses, this PR also refactored them so that they're no longer exported, and the tests no longer do string comparison, as discussed in this issue. Lint issues for trace are also resolved in this PR.

[the-ericwang35]

The lint errors are unrelated to this PR (we just added golint to the CI which is catching lint errors from before). See https://github.com/googleinterns/cloud-operations-api-mock/issues/30