feat(cron): Support multiple cron expressions

[irvinlim]

Closes #7.

Overview

This PR introduces support for multiple cron expressions per JobConfig.

For example, given the following JobConfig spec:

spec:
  schedule:
    cron:
      expressions:
        - '0/30 10-19 * * *'
        - '0 0/2 * * *'

This allows us to specify that the JobConfig shall be periodically scheduled as follows:

1. Every 30 minutes, starting at 10:00 and ending at 19:30.
2. Every 2 hours outside of 10:00-20:00 (i.e. 00:00, 02:00, ..., 08:00, 20:00, 22:00)

API Semantics

1. At most one of expression/expressions is allowed in spec.schedule.cron.
2. The order of cron expressions does not matter.
3. When multiple cron expressions fall on the same time, only one of them will trigger a schedule. In the above example, 10:00, 12:00, ... are covered by both 0/30 10-19 * * * and 0 0/2 * * *. However, the JobConfig will be scheduled only once.

Changes

1. Adds a new field expressions to spec.schedule.cron and make both expression/expressions optional.
2. Update validation webhook to validate new field.
3. Add the Expression interface, which is implemented by *cronexpr.Expression, as well as a new multiExpression implementation.
4. Support multiExpression which returns the earliest next value for Next().
5. Update CLI to display multiple cron expressions correctly.

[codecov]

Codecov Report

Base: 70.36% // Head: 70.60% // Increases project coverage by +0.24% 🎉

Coverage data is based on head (c08ae2c) compared to base (c8e12c6).
Patch coverage: 82.60% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
+ Coverage   70.36%   70.60%   +0.24%     
==========================================
  Files         194      200       +6     
  Lines       10284    10386     +102     
==========================================
+ Hits         7236     7333      +97     
- Misses       2661     2664       +3     
- Partials      387      389       +2     

Impacted Files	Coverage Δ
pkg/execution/util/cronschedule/option.go	100.00% <ø> (ø)
pkg/execution/variablecontext/provider.go	55.10% <0.00%> (-3.60%)	⬇️
pkg/cli/format/schedule.go	37.93% <37.93%> (ø)
pkg/execution/util/cron/util.go	53.84% <53.84%> (ø)
pkg/execution/util/cronschedule/schedule.go	77.32% <85.71%> (ø)
pkg/cli/common/config.go	88.88% <88.88%> (ø)
pkg/cli/cmd/cmd_get_jobconfig.go	79.71% <91.66%> (+4.40%)	⬆️
pkg/cli/format/time.go	85.00% <91.66%> (+2.64%)	⬆️
pkg/execution/util/cron/parser.go	79.31% <91.66%> (ø)
apis/execution/v1alpha1/jobconfig_types.go	100.00% <100.00%> (ø)
... and 13 more

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