feat: Make gcs and s3 credentials optional

[scsmithr]

Closes #991

~/Code/github.com/glaredb/glaredb [2] % cargo run --bin glaredb -- local
    Finished dev [unoptimized + debuginfo] target(s) in 0.58s
     Running `target/debug/glaredb local`
GlareDB (v0.0.11)
Using in-memory catalog
> create external table v from gcs options (location = 'tyrell/voight_kampff.csv', bucket='glaredb-demos');
create table
> select * from v limit 10;
+--------------------------------------+---------------------+--------------------+-----------------+------------------+------------+
| Test_ID                              | Examiner_Name       | Subject_Name       | Questions_Asked | Final_Assessment | Test_Date  |
+--------------------------------------+---------------------+--------------------+-----------------+------------------+------------+
| 273886b2-c778-4757-8779-68cf4de40154 | Angelica Wilson     | Eric Jarvis        | 68              | Replicant        | 2020-01-01 |
| b9831559-803c-40b7-bf12-b21417ed6bcd | Robert Hall         | Margaret Smith     | 71              | Human            | 2020-01-02 |
| 38de6ce6-4012-4e0f-99b1-a2f4757fe8b9 | Victoria Sims       | Mrs. Sheila Hall   | 83              | Human            | 2020-01-03 |
| 2c36ad83-7049-4cfc-ba51-af96f1c2eebe | Shannon Horton      | Andrew Wiggins     | 82              | Human            | 2020-01-04 |
| 1b8827c1-cd9b-4fa6-8fc0-e7b402acc184 | Sheila Torres       | Mark Tate          | 84              | Human            | 2020-01-05 |
| 6cd8ee64-5ff2-475b-b276-c2bda44f6330 | Nicole Taylor       | Justin Fleming     | 47              | Replicant        | 2020-01-06 |
| 5b04797c-e861-41eb-a76d-12e7c1d3e6c0 | Patrick Harrison MD | Laura Hicks        | 22              | Human            | 2020-01-07 |
| e477d5e3-f886-4d5c-8ba4-b403513450fc | Edward Salinas      | Brittany Hendricks | 101             | Replicant        | 2020-01-08 |
| 0be01ad5-9359-48e6-85a7-38ca0abf9276 | Michael Nelson      | Betty Lawson       | 40              | Replicant        | 2020-01-09 |
| c4c59f2d-c94a-4f3b-bc0e-9a727f889bec | Susan Chandler      | Debbie Weeks       | 45              | Replicant        | 2020-01-10 |
+--------------------------------------+---------------------+--------------------+-----------------+------------------+------------+