[SEDONA-379] Fix bugs in RS_AsBase64

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read Contributor Rules and Contributor Development Guide

Is this PR related to a JIRA ticket?

• Yes, the URL of the associated JIRA ticket is https://issues.apache.org/jira/browse/SEDONA-XXX. The PR name follows the format [SEDONA-XXX] my subject.

What changes were proposed in this PR?

1. Rename the function to RS_AsBase64
2. Fix the wrong name of RasterOutputTests. This stopped JUnit from picking up the tests when run with mvn install
3. Use another image for the test as sometimes (for some images) Java8 and Java11 produce different base64 strings given the same input image.
4. Improve the documentation to explain how this function works. In particular, this function only visualizes the first 4 bands regardless how many bands the input raster could have.

How was this patch tested?

Passed unit tests

Did this PR include necessary documentation updates?

• Yes, I have updated the documentation update.