appender: add a builder for constructing RollingFileAppenders

[hawkw]

Motivation

Several currently open PRs, such as #2225 and #2221, add new
configuration parameters to the rolling file appender in
tracing-appender. The best way to add new optional configuration
settings without breaking existing APIs or creating a very large number
of new constructors is to add a builder interface.

Solution

Since a number of PRs would all need to add the builder API, introducing
potential conflicts, this branch just adds the builder interface
without adding any new configuration options. Once this merges, the
existing in-flight PRs can be rebased onto this branch to use the
builder interface without conflicting with each other.

Also, the Builder::build method is fallible and returns a Result,
rather than panicking. This is a relatively common pattern in Rust ---
for example, std::thread::Builder::spawn returns a Result if a new
thread cannot be spawned, while std::thread::spawn simply panics. This
allows users to handle appender initialization errors gracefully without
breaking the API of the existing new constructor.

Fixes #1953