Support union and none type in flyteidl

[yubofredwang]

TL;DR

Union type is not supported in flyteidl leading to flytectl failures when the task takes Union type as input or output.
When users do operations such as:
flytectl get execution
will output:
failed to convert to a known Literal. Input Type [union_type:..] not supported

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

For creating making literal for union type, the code will find the first one that matches the input value. If the input is 1.0
and union type is Union(Integer, Float), integer will be used since 1.0 will match with integer.

For default generation of a union type, I just use the first variants possible. For example, if the union contains
Union(String, Integer), then default value generated when creating an execFile will be "" not 0.

Tested with flytectl commands, get task and create execution is working fine

Tracking Issue

fixes flyteorg/flyte#3094

[codecov]

Codecov Report

Merging #401 (2441d95) into master (44ae401) will increase coverage by 2.51%.
The diff coverage is 82.22%.

❗ Current head 2441d95 differs from pull request most recent head 4077783. Consider uploading reports for the commit 4077783 to get more accurate results

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   76.11%   78.62%   +2.51%     
==========================================
  Files          18       18              
  Lines        1390     1240     -150     
==========================================
- Hits         1058      975      -83     
+ Misses        280      209      -71     
- Partials       52       56       +4     

Flag	Coverage Δ
unittests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
clients/go/coreutils/extract_literal.go	85.48% <71.42%> (+1.27%)	⬆️
clients/go/coreutils/literals.go	92.91% <84.21%> (+0.76%)	⬆️

... and 16 files with indirect coverage changes