Allow users to explicitly mark keys as not eligible for Garbage Collection

Is your feature request related to a problem? Please describe

In managed mode, a user may want to mark keys as not eligible for garbage collection.

With the following conditions:

• KeepNumVersions=MaxInt64
• DiscardTs > KeyTs
• deleted or expired key at a higher Ts

A key will be eligible for garbage collection.

Describe the solution you'd like

Expose a method in Entry that allows users in Managed Mode to set a Never Discard bit.

// WithNeverDiscard sets merge bit in entry's metadata. This
// prevents the compactor from marking the key eligible for GC.
func (e *Entry) WithNeverDiscard() *Entry {
	e.meta = bitMergeEntry // this works -- but could use another bit
	return e
}

Describe alternatives you've considered

Allow users to explicitly mark keys eligible for garbage collection when DiscardTs is unset.

Additional context

This is useful when users want to leverage the Multi-Version system of BadgerDB in Managed Mode to maintain multiple versions of keys.

Start by tracing Entry metadata handling and the compactor logic that marks keys eligible for garbage collection in managed mode. Review the existing merge-bit behavior and determine how a never-discard marker should interact with the listed version and timestamp conditions. Done means users can set the marker through Entry and those keys are excluded from GC, with coverage for the described case.