Allow 'auto' to be passed as a height

[Whatshiywl]

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
The headerHeight and footerHeight inputs of ngx-datatable accept the 'auto' value as well as numbers, going as far as verifying if their values match that specific string. However, their current type signature allows for numbers only. This type conflict pops up when using strictTemplates as an Angular Compiler Option.

What is the new behavior?
The two aforementioned inputs will now allow the 'auto' value to be passed without incurring in build errors when template type checking is enabled.

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

[achimha]

if headerHeight/footerHeight is 'auto', the calculation with height = height - (this.footerHeight as number) will not work.