[PoC] Split EnabledParameters into read-write and read-only types

[MrAlias]

This is a proof-of-concept for refactoring the EnabledParameters into a read-only type. A EnabledParametersBuilder type is added to act as a read-write type that wraps EnabledParameters.

In response to #5816 (comment)

The basic idea is a user will create a EnabledParametersBuilder and pass the built EnabledParameters to the Logger.Enabled method:

epb := new(log.EnabledParametersBuilder)
epb.SetSeverity(log.SeverityInfo)
_ = logger.Enabled(ctx, epb.Build())

The EnabledParametersBuilder type can be reused as it passes a static copy of EnabledParameters every time Build is called.

This means that any Logger, and subsequently any Processor in the SDK, will receive a read-only type. If they wish to update this type they can always create a new copy of EnabledParameters by doing something like this:

func (p *Processor) Enabled(ctx context.Context, ep log.EnabledParameters) bool {
	epb := log.EnableParametersBulider{ep}
	epb.SetSeverity(log.SeverityError)
	return downstream(ctx, epb.Build())
}

[MrAlias]

Closing, not needed: #5816 (comment)