Replace base64 URL encode/decode APIs

[ldclakmal]

Purpose

This PR replaces the usages of base64 URL encode/decode APIs from ballerina/encoding module.

Fixes: ballerina-platform/ballerina-library#1212

Related to: ballerina-platform/ballerina-library#907

Related to: ballerina-platform/ballerina-library#584

[codecov]

Codecov Report

Merging #129 (3605646) into master (98a55f3) will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #129      +/-   ##
============================================
- Coverage     70.63%   70.56%   -0.07%     
- Complexity       93       97       +4     
============================================
  Files            10       11       +1     
  Lines           538      547       +9     
  Branches        272      272              
============================================
+ Hits            380      386       +6     
- Misses          155      158       +3     
  Partials          3        3              

Impacted Files	Coverage Δ	Complexity Δ
.../java/org/ballerinalang/stdlib/jwt/JwksClient.java	63.49% <0.00%> (+1.95%)	10.00 <0.00> (ø)
...in/java/org/ballerinalang/stdlib/jwt/JwtUtils.java	44.44% <44.44%> (ø)	2.00 <2.00> (?)
jwt-ballerina/jwt_issuer.bal	70.52% <60.00%> (ø)	13.00 <1.00> (ø)
jwt-ballerina/jwt_validator.bal	78.00% <66.66%> (ø)	63.00 <3.00> (ø)
jwt-ballerina/jwt_commons.bal	100.00% <100.00%> (ø)	2.00 <2.00> (+2.00)

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