Add concurrency for tag/digest reading

[TerryHowe]

Add concurrency for reading tags/digests similar to the tagging concurrency.

Scan and resolve every tag in the list will degrade the performance. Below are some experiments I made based on the current implementation:

oras repo tags docker.io/library/alpine | wc
time oras repo tags docker.io/library/alpine:latest
oras repo tags mcr.microsoft.com/cbl-mariner/distroless/minimal | wc
time oras repo tags mcr.microsoft.com/cbl-mariner/distroless/minimal:1.0

On my computer , it takes DockerHub 47.442 seconds to scan 129 tags, and MCR 19.192 seconds to scan 141 tags.

Originally posted by @qweeah in oras-project/oras#800 (comment)

[shizhMSFT]

@TerryHowe Could you elaborate more on this feature request?