Rename BasicAuth record to Credentials

[Maninda]

Purpose

Resolves ballerina-platform/ballerina-library#1606

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #310 (79994fd) into master (6d37e8b) will not change coverage.
The diff coverage is 75.00%.

@@            Coverage Diff            @@
##             master     #310   +/-   ##
=========================================
  Coverage     84.63%   84.63%           
  Complexity       82       82           
=========================================
  Files            17       17           
  Lines           768      768           
  Branches        108      108           
=========================================
  Hits            650      650           
  Misses           92       92           
  Partials         26       26           

Impacted Files	Coverage Δ
ftp-ballerina/commons.bal	100.00% <ø> (ø)
...rg/ballerinalang/stdlib/ftp/util/FtpConstants.java	100.00% <ø> (ø)
...ava/org/ballerinalang/stdlib/ftp/util/FtpUtil.java	79.77% <75.00%> (ø)

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