feature: acquire exact tarantool version

[Totktonada]

Sometimes we want to test a module against specific tarantool version,
not a latest in the series. For example, 2.6 series may have 2.6.1,
2.6.2 and 2.6.3 releases and we want to setup 2.6.1.

Fixes #15

---

Usage examples:

steps:
  - uses: actions/checkout@v2
  - uses: tarantool/setup-tarantool@v1
    with:
      tarantool-version: '2.8.1'

(Before the patch we can only choose tarantool-version: '2.8' and the latest version in the 2.8 series (2.8.4) would be installed. Now we can choose one of previous versions in the series.)

steps:
  - uses: actions/checkout@v2
  - uses: tarantool/setup-tarantool@v1
    with:
      tarantool-version: '1.10.13.23'
      nightly-build: true

(We can also write tarantool-version: '1.10.13' to install the latest nightly build of 1.10.13 -- it is 1.10.13.23 at the moment of writing.)

[Totktonada]

Tested on tarantool/smtp: tarantool/smtp@20b2404.

[DifferentialOrange]

To be honest, TypeScript part seems easy to understand even for a first-timer, but GH Actions part left me a bit confused. I have left a couple of questions, but if I got it right this PR should be ok