structured logging: add Http request context discovery

[minherz]

Is your feature request related to a problem? Please describe.
If not defined, the httpRequest field in LogEntry should be auto-populated when writing log entries to backend

Describe the solution you'd like
Provide a method to define an Http context that can be used by a Web server application to publish info about the request context and also to remove the published context.
The method should support multi-threading. It should be available from any thread of the application process.

[minherz]

Related issues #685, #686, #687, #688, #689, #690, #691

[minherz]

The Spring context can be discovered using the method recommended in the stackoverflow post.

[minherz]

This feature covers two implementations:

1. The request context holder that will maintain the context per-thread providing support to existing Web server implementations.
2. The ServletContainerInititializer implementation that is designed as a sub-module in this repository, so its dependencies to javax-servlet-api will not cause backward incompatability to the google-cloud-logging submodule.

[minherz]

Closing parent issue due to completing both child issues.