Explicitly use string as type for key in cache package

[nghialv]

What this PR does / why we need it:

This simplifies the cache interface.

Which issue(s) this PR fixes:

Fixes #2199

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 32.07%. This pull request does not change code coverage.

[nghialv]

/changelog

[pipecd-bot]

@nghialv: Changelog has been generated in response to this comment.

Details

Changelog since v0.10.3

Notable Changes

• Introducing plan-preview feature (#2195)
• Fix wrong drift result that caused in v0.10.3 (#2192)

Internal Changes

• Store reported piped stats to redis (#2190)
• Fix doc titles (#2198)
• Do not render json files under grafana-dashboards (#2197)
• Enable plan-preview in piped by default (#2194)
• Add minimum required dashboards (#2115)
• Put Grafana dashboards in different folders (#2191)
• Make terraform cloud provider able to parse log including ansi codes (#2189)
• Add relabel config to plot durations for each gRPC method (#2187)
• Create a dedicated gitClient for plan-preview (#2188)
• Use custom piped metrics registry instead of the default one (#2183)
• Fix Envoy config to be valid as an extension (#2186)
• Enable Envoy gRPC stats for all methods (#2185)
• Fix wrong generated URLs for application, env and piped (#2184)
• Introducing ReportStat service to pipedservice (#2181)

[khanhtc1202]

💯
/lgtm

[nakabonne]

They should 👍
/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.