dialogue - In Tombstone what is the meaning of the line spoken by Doc Holiday?

The Dictionary of American Slang defines this, and supplies just this line from Val Kilmer in Tombstone as an example of its usage.

"I'm your huckleberry" means "I'm just the man you're looking for!"

"I'm your huckleberry..." Val Kilmer as Doc Holliday in Tombstone

Also, the podcast A Way With Words from Public Radio, is a call-in show about linguistics, and answered this question this way:

What it means is easy enough. To be one’s huckleberry — usually as the phrase I’m your huckleberry — is to be just the right person for a given job, or a willing executor of some commission. Where it comes from needs a bit more explaining...