feat(GlDatepicker): add `placeholder` and `autocomplete` props

What does this MR do?

Adds placeholder and autocomplete props to GlDatepicker so that you can set the placeholder and autocomplete properties on the input.

autocomplete defaults to off if the datepicker opens on focus because autocomplete dropdown conflicts with the datepicker.

Custom placeholder

Autocomplete issue

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released
  • New props, shouldn't be needed
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] Security reports checked/validated by a reviewer from the AppSec team