[FEATURE] Support PPL getExemplar API

[YANG-DB]

Is your feature request related to a problem?
I would like to add a ppl function to load exemplar from a metrics

What solution would you like?

Add getExemplar PPL function getting exemplars from a metrics using the next attributes:

• metrics name
• start date
• end date

This would return all the exemplars within that metrics time bounds.

In the Prometheus datasource this API would delegate to this API for accessing the metrics exemplars

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?

• https://prometheus.io/docs/prometheus/latest/querying/api/#querying-exemplars
• https://github.com/opensearch-project/sql/tree/main/prometheus

[vamsi-amazon]

Will start coding and design Today.